A decorator that drives a whole cycle of the inner stage per outer run — like Repeat, on the same outer input — folding every output the inner stage produces into an accumulator with the binary operation stage op.

The first output becomes the accumulator without invoking op (the classic seedless reduce); each subsequent output is folded via op((accumulator, output)). In an iteration with nothing to fold — the inner stage produced no output, or the output is the first one — op is skipped, so each iteration performs exactly one of apply/skip on both inner stages, as the lifecycle contract requires. An iteration yielding no output with h8io.stages.Status.Success keeps the cycle spinning, like Repeat. The cycle is @tailrec and does not grow the stack.

When op is applied but itself yields no output for a given fold — e.g. it filters some outputs out — the accumulator is left unchanged rather than discarded, exactly as in Fold. op's status is still combined into the iteration's status in this case, since op was applied, not skipped, for that output.

The cycle stops when the status of an iteration is h8io.stages.Status.Complete: an error-free Complete is reported to the enclosing pipeline as Success, a Complete with errors is preserved. The value accumulated so far, if any, is yielded. The accumulator is local to one outer run: the next generation of Reduce starts empty.

The inner pipeline

alterand and op together form the inner pipeline of Reduce, with op downstream of alterand. An iteration's status is therefore the combination of their statuses in pipeline order (alterand first), and both continuations are selected by that combined status — exactly as h8io.stages.Evolution.AndThen evolves both sides of a composed pipeline with the status of the whole run, never with the stages' individual statuses.

As in Loop and Repeat, the continuations are never selected by the status Reduce reports to the enclosing pipeline — the two differ, since an error-free Complete is absorbed into a Success — and for the same reason the returned evolution is a h8io.stages.base.ConstEvolution. Its dispose releases both inner evolutions via h8io.stages.Evolution.dispose (op first, then alterand), which per the h8io.stages.Evolution contract stay the terminal cleanup handles of the generation just constructed. Symmetrically a skipped Reduce skips both inner stages and selects their continuations on the neutral Status.Success.